1. Field of the Invention
The present invention relates to delta-sigma modulators.
2. Description of Related Art
As depicted in FIG. 1, a delta-sigma modulator (DSM) 100 receives an analog input signal and converts it into a digital output signal at a sampling rate appreciably higher than the bandwidth of the analog input signal. DSM 100 comprises a digital-analog converter (DAC) 104 for converting the digital output signal into an analog feedback signal, a subtraction circuit 101 for generating an error signal representing a difference between the analog input signal and the analog feedback signal, a loop filter 102 for filtering the error signal and outputting a filtered signal, and a quantizer 103 for quantizing the filtered signal into the digital output signal. DSM 100 spectrally shapes the quantization noises of the quantizer 103 so that the quantization noise appears as mostly high frequency noise in the digital output signal. As long as the sampling rate is sufficiently higher than the bandwidth of the analog input signal, the spectrally shaped quantization noise components are mostly beyond the bandwidth of the analog input signal and thus can be filtered out. This allows high resolution analog-digital conversion without using a high resolution quantizer. Principles and theories of DSM are well known in prior art and thus not explained in detail here. The quantizer 103 can be either a single-bit quantizer or a multi-bit quantizer. A multi-bit quantizer offers many advantages over a single-bit quantizer. However, when quantizer 103 is a multi-bit quantizer, DAC 104 also needs to be a multi-bit DAC, because DAC 104 must convert the digital output signal generated by quantizer 103 into the analog feedback signal. A practical multi-bit DAC is not perfectly linear, and its nonlinearities contribute to errors to the delta-sigma modulator 100 and thus degrade the performance.
Dynamic element matching techniques, e.g. DWA (data weighted averaging), can be used to spectrally shape the errors caused by the nonlinearities of the multi-bit DAC so that the errors also appear as high frequency noise. While dynamic element matching techniques are very effective in suppressing the static errors caused by the nonlinearities of the multi-bit DAC, they rely on dynamic and frequent changes in utilizing the constituent elements of the multi-bit DAC. This unfortunately may lead to an increase in dynamic errors that can degrade the performance of the delta-sigma modulator.
What is desired is a sigma-delta modulator that overcomes the shortcomings of the prior art.